CD4+ T lymphocytes are the main cells responding to Mycobacterium tuberculosis antigens. (a) Proliferative responses ([PKH]-26 assay) to purified protein derivative (PPD) and to culture filtrate protein (CFP)-10 are shown for two individuals, one without (no tuberculosis [TB]) and the other with a previous or latent TB. Dot plots are gated on CD3+ cells and analyse the fluorescence intensity for both CD4 and PKH-26. Proliferating cells are PKH-26low. The percentage of CD3+ cells in each quadrant is shown. (b) The release of interferon (IFN)-γ in response to PPD, CFP-10, or phytohemagglutinin (PHA) was tested using the enzyme-linked immunosorbent spot assay, evaluating the response of either unfractionated peripheral blood mononuclear cells (PBMCs) (left) or PBMCs depleted of CD4+ T lymphocytes (right). Results are from two individuals with previous or latent TB and are expressed as the number of IFN-γ-releasing cells per million cells.